1. Field of the Invention
The present invention relates to filtration media comprising one or more layers of nanofibers. The filtration media are especially suitable for filtering contaminants from liquids.
2. Background
Liquid filtration media are generally categorized in one of two types. One type is fibrous nonwoven media, including but not limited to spunbonded, melt blown or spunlaced nonwoven media formed from continuous fibers, hydroentangled nonwoven media formed from carded staple fiber, and media which is made from a combination of the above technologies. Nonwoven media for liquid filtration have pore sizes greater than 1 μm. The second type of liquid filtration media is porous film membranes which can be used unsupported or in conjunction with a support layer. Filtration membranes have pore sizes less than 0.01 μm and are typically used to accomplish fine filtration tasks such as microfiltration in which particulates in the range of about 0.1 μm to about 10 μm are filtered from the liquid, ultrafiltration in which particulates in the range of about 50 nm to about 0.5 μm are filtered and reverse osmosis in which matter in the range of about 1 Å to about 1 nm is filtered. Nonwoven media and membranes are both suitable for use in microfiltration.
In order to achieve particle retentions equivalent to pore sizes of less than 1 μm with nonwoven media, nonwoven media has been made with increased depth by increasing the number of layers of fibrous material. This desirably results in increased tortuosity of a defect path through which a contaminant particle would have to pass in order to escape capture by the medium and increased contaminant-holding capacity of the medium. Increasing the number of fibrous layers in nonwoven media also undesirably increases the pressure drop or differential pressure across the media in use, which translates to increased energy for the filter user and shorter filter life. Membranes, on the other hand, can offer a combination of good particle retention, pressure drop and flux, but tend to be cost-prohibitive, and do not provide good contaminant-holding capacity over the entire range of pressure drop, therefore limiting the life of filters using membranes.
U.S. Patent Publication Number 20040038014 A discloses a nonwoven filtration medium for filtering contaminants from air or liquids, the medium comprising a single layer or multiple layers of fine fibers. The fine fiber is formed by an electrostatic spinning process utilizing an apparatus including an emitting device and a grid having a high voltage electrostatic field maintained therebetween. A solution of a polymer in a solvent is delivered to the emitting device from which a droplet of the solution is accelerated by the electrostatic field toward the grid, whereupon the solvent evaporates and the polymer is drawn into a fine fiber and collected as a dry fiber on a collection substrate located between the emitting device and the grid.
Synthetic polymers have been formed into webs of very small diameter fibers, i.e., on the order of a few micrometers or less than 1 μm, using various processes including melt blowing, electrostatic spinning and electroblowing. Such webs are useful as liquid barrier materials and filters. Often they are combined with stronger sheets to form composites, the stronger sheets providing the strength to meet the needs of the final filter product.
It would be desirable to have a nonwoven filtration medium suitable for filtering contaminants from liquids that provides an improved combination of pressure drop and flow rate. It is believed that such a medium would provide increased filter longevity.